1. Field of the Invention
The present invention relates to a protective element wherein a heat-generating member generates heat under abnormal circumstances, which causes a low melting metal member to blow out.
2. Description of the Related Art
Protective elements comprising heat-generating members and low melting metal members layered on a substrate are known to be able to prevent overvoltages as well as overcurrents (e.g. Japanese Patent No. 2790433 and Japanese Patent Application Laid-open No. H8-161990). Such protective elements contain heat-generating members through which electricity is passed in abnormal circumstances, and the heat generated by these members melts the low melting metal. The surface of the electrode on which the low melting metal member is disposed is thereby wetted, which causes the low melting metal member to blow out.
FIG. 1 is a circuit diagram showing an example of an overvoltage-preventing device which utilises such a protective element 1p. FIGS. 2A and 2B shows respectively a plane view and a sectional view of the protective element 1p. 
The protective element 1p has a structure comprising heat-generating members 3, on which a resist paste has been applied, an insulating layer 4, and a low melting metal member 5 comprising a fuse material, all of which are layered on a substrate 2. In the drawings, 6a and 6b are electrodes for the heat-generating members; the electrode 6b is connected to an electrode in the central portion of the low melting metal member 5 (intermediate electrode 7c), with the connection site being situated between two sites 5a and 5b into which the low melting metal member 5 has been divided. 7a and 7b are electrodes for the low melting metal member. 8 is a internal sealing component made from solid flux, with which the low melting metal member 5 is sealed to prevent its surface from oxidising, and 9 is an external sealing component comprising a material which has higher melting and softening points than the low melting metal member 5 and which prevents the molten low melting metal member 5 from flowing out of the protective element once it has blown out.
In the overvoltage preventing device shown in FIG. 1, which uses the protective element 1p, terminals A1 and A2 are connected to electrode terminals on the device to be protected; e.g. a lithium-ion battery, while terminals B1 and B2 are connected to electrode terminals of devices such as a charging device connected to the device to be protected. According to the overvoltage preventing device, when the charging of the lithium-ion battery proceeds and an overvoltage that is larger than the breakdown voltage is applied to a Zehner diode D, an abrupt base current ib will flow and cause a large collector current ic to flow through the heat-generating members 3 and thereby cause the heat-generating members 3 to heat up. This heat is transmitted to the low melting metal member 5 on the heat-generating members 3, causing the two sites 5a and 5b of the low melting metal member 5 to blow out. This prevents any overvoltage from being applied to the terminals A1 and A2 and simultaneously interrupts the current flowing to the heat-generating members 3.
An example of an embodiment of connection between the low melting metal member and heat-generating member in such types of protective elements is taught in Japanese Patent Application Laid-Open No. H10-116549 and Japanese Patent Application Laid-Open No. H10-116550, wherein the low melting metal member and the heat-generating member are connected while being disposed in planar fashion on the substrate, instead of the low melting metal member being layered on top of the heat-generating member. Nevertheless, the merit of having the flow of electricity to the heat-generating member being interruptible at the same time that the low melting metal member blows out remains the same.
Similarly to the protective element 1p shown in FIG. 1, FIGS. 3A, 3B and 3C depict respectively a plane view (3A) and sectional views (3B, 3C) of a protective element 1q, in which the heat generated from a flow of electricity passing through a heat-generating member 3 causes a low melting metal member 5 to blow out and the flow of electricity destined for the heat-generating member 3 to be simultaneously interrupted (Japanese Patent Application No. H11-110163). Low melting metal member electrodes 7a, 7b and 7c are furnished on a substrate 2 in this protective element 1q, and a low melting metal member 5 (5a, 5b) is disposed so as to bridge these electrodes 7a, 7b and 7c. A heat-generating member 3 is furthermore furnished on the underside of the electrode 7c, with an insulating layer 4 interposed therebetween. The heat-generating member 3 is heated by the flow of electricity passed between the heat-generating member electrode 6b and the leads 6x and 6y coming from the heat-generating member electrode 6a. The heat-generating member electrode 6b is connected to the low melting metal member electrode 7c. Accordingly, the heat generated by the heat-generating member 3 causes both the low melting metal member 5a between the electrodes 7a and 7c and the low melting metal member 5b between the electrodes 7b and 7c to blow out, and thereby interrupt the flow of electricity passed to the device to be protected, while also interrupting the flow of electricity transmitted to the heat-generating member 3.
However, even if the heat generated by the heat-generating member 3 melts the low melting metal member 5 in the aforedescribed conventional protective elements 1p and 1q, the drawback arises that if the surface areas of the low melting metal member electrodes 7a, 7b and 7c are too narrow, the molten low melting metal member 5 will not flow sufficiently onto these electrodes and the low melting metal member 5 shall not blow out.
The drawback will also arise, moreover, that if the distance between adjacent electrodes (inter-electrode distance) with the low melting metal member 5 interposed therebetween is too small, the low melting metal member will not blow out despite being melted by the heat generated by the heat-generating member 3 has melted it. Conversely, if the inter-electrode distance is too great, the heat generated when the low melting metal member 5 has been connected to the substrate 2 will create a drawback that the low melting metal member 5 will become thin in specific areas, and moreover, the anti-pulse property will diminish whether or not the resistance value remains constant. Last, another drawback will arise when the inter-electrode distance is further increased, as the low melting metal member 5 will blow out when it is bonded to the substrate by thermocompression or the like.
With the foregoing problems in view, it is an object of the present invention to provide a protective element comprising a heat-generating member and a low melting metal member on a substrate, in which the low melting metal member is molten by the heat generated by the heat-generating member and flows onto electrodes, the low melting metal member being thereby caused to blow out, wherein the manufacturing stability and reliability of the protective element is enhanced by optimising both the surface area and the inter-electrode distance of the electrodes to which the molten low melting metal member flows when the heat is generated by the heat-generating member.
In achieving the aforedescribed object, the present invention provides a protective element comprising a heat-generating member and a low melting metal member on a substrate, in which the low melting metal member is molten by the heat generated by the heat-generating member and flows onto electrodes, whereby the low melting metal member is blown out, the protective element being characterised in that Formula (1) below is satisfied for at least one of the electrodes onto which the molten low melting metal flows:
(cross-sectional area of the low melting metal member)/(blowout effective electrode surface area)xe2x89xa60.15xe2x80x83xe2x80x83(1) 
where the cross-sectional area of the low melting metal member is defined as an average value of the cross-sectional areas of the planes established perpendicularly with respect to the direction of the current which flows through the low melting metal member, and the blowout effective electrode surface area is defined as the surface area of the electrodes which can be wetted by the molten low melting metal member which flows onto the electrodes, in one minute after the low melting metal member has been completely molten and started to flow. The present invention further provides a protective element comprising a heat-generating member and a low melting metal member on a substrate, in which the low melting metal member is molten by the heat generated by the heat-generating member and flows onto electrodes, whereby the low melting metal member is blown, the protective element being characterised in that Formula (2) below is satisfied:
2.5xe2x89xa6(inter-electrode distance)/(cross-sectional area of the low melting metal member)xe2x89xa630xe2x80x83xe2x80x83(2) 
where the cross-sectional area of the low melting metal member is defined as an average value of the cross-sectional areas of the planes established perpendicularly with respect to the direction of the current which flows through the low melting metal member, and the inter-electrode distance is defined as the distance between two electrodes, from among the electrodes onto which the molten low melting metal member flows, which are adjacent to each other with the low melting metal member interposed therebetween.
The cross-sectional area of the low melting metal member, as described in the foregoing, is defined as the cross-sectional area of the low melting metal member at a surface which is established perpendicularly to the direction of the current which flows through the low melting metal member. Should the cross-sectional area, however, not be constant with respect to the direction of the current flowing through the low melting metal member, then it should be defined as the value obtained by taking an average of the cross-sectional areas in the direction of the current.
The blowout effective electrode surface area is defined as the surface area of the electrode which is wetted by the low melting metal member one minute after it has completely melted and started to flow. When there is a plurality of electrodes which the molten low melting metal member is able to wet, then Formula (1) should be satisfied for at least one of the electrodes.
The blowout effective electrode surface area is normally equal to the total surface area of the electrodes onto which the molten low melting metal member flows. However, when the total surface area of the electrodes is greater than the surface area which can be wetted by the molten low melting metal member one minute after it starts to flow, then the blowout effective electrode surface area shall be defined as a portion of the total electrode surface area.
The protective element pertaining to the present invention is formed so as to satisfy the aforedescribed Formulae (1) and (2), which enables its reliability as a protective element to be improved, since the heat generated by the heat-generating member will cause the low melting metal member to blow out quickly. The manufacturing stability of the protective element can also be enhanced accordingly.